* ref(large-video): reactify background

This is pre-requisite work for disabling the background on
certain browsers, namely Firefox. By moving the component
to react, and in general encapsulating background logic,
selectively disabling the background will be easier.

The component was left for LargeVideo to update so it can
continue to coordinate update timing with the actual large
video display. If the background were moved completely into
react and redux with LargeVideo, then background updates would
occur before large video updates causing visual jank.

* fix(large-video): do not show background for Firefox and temasys

Firefox has performance issues with adding filter effects on
animated elements. On temasys, the background videos weren't
really displaying anyway.

* some props refactoring

Instead of passing in classes to LargeVideoBackground, rely on
explicit props. At some point LargeVideo will have to be reactified
and the relationsihp between it and LargeVideoBackground might
change, so for now make use of props to be explicit about
how LargeVideoBackground can be modified.

Also, set the jitsiTrack to display on LargeVideoBackground to
null if the background is not displayed. This was an existing
optimization, although previously done with pausing and playing.

* squash: use newly exposed RTCBrowserType

* squash: rebase and use new lib browser util

* squash: move hiding logic all into LargeVideo

* squash: remove hiding of background on stream change. hopefully doesnt break anything

As an intermediate step on the path to merging jitsi-meet and
jitsi-meet-react, import the whole source code of jitsi-meet-react as it
stands at
2f23d98424
i.e. the lastest master at the time of this import. No modifications are
applied to the imported source code in order to preserve a complete
snapshot of it in the repository of jitsi-meet and, thus, facilitate
comparison later on. Consequently, the source code of jitsi-meet and/or
jitsi-meet-react may not work. For example, jitsi-meet's jshint may be
unable to parse jitsi-meet-react's source code.